Imide prepolymers are prepared through reaction between unsaturated bisimides such as N,N'-4,4'-diphenylmethanebismaleimide and diamines such as diaminodiphenylmethane by mixing them and heating the mixture or by heating them in high-boiling solvents such as N-methylpyrrolidone. These imide prepolymers are heat cured into products having high heat resistance and a low coefficient of thermal expansion. For these characteristics, the imide prepolymers have heretofore found major use as resins for multilayer printed circuit boards. Regretfully, cured products of these imide prepolymers are less flame retardant For example, laminates using the imide prepolymers fail to meet the flame retardance rating V-0 when subjected to a burning test according to UL-94.
In recent years, severe flame retardance requirements are imposed on electronic, automobile and other parts used in general living utilities for safety against fire. The resins which can be used in some areas have to meet the flame retardance requirement prescribed by laws or rules.
One typical method for imparting flame retardance to imide prepolymers is to modify imide prepolymers with brominated epoxy resins. Although this method is successful in imparting flame retardance, the modified prepolymers suffer from a marked loss of high-temperature properties, especially soldering heat resistance when molded into laminates. There is a need for heat resistant resins having high flame retardance and high-temperature properties in laminate form.